This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-139677, filed May 20, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a distance-measuring system used in an image pickup device, such as a silver bromide camera, a digital camera, or a video camera, and more particularly to a distance-measuring system for measuring subject distances at points on the shooting screen (so-called multi-autofocus, hereinafter referred to as multi-AF), characterized by a wide distance-measuring area, such as full-screen AF for measuring
Today, it is becoming quite common for image pickup devices, such as cameras, to implement multi-AF. Cameras equipped with a distance-measuring device for measuring subject distances at three, five, or seven points on the shooting screen are commercially available in low-price models.
The multi-AF is one-dimensional multi-AF where distance-measuring areas are arranged on a straight line. Recently, there have been signs of the commercialization of two-dimensional multi-AF or area AF.
For example, a camera equipped with a distance-measuring device having an area AF function using as many as 45 distance-measuring areas 17 on the finder view 16 has been commercialized and put on the market.
In such conventional multi-AF, complex calculations, such as distance-measuring calculations, must be repeated as many times as the number of distance-measuring areas increases. To improve the time-lag, various inventions have been disclosed.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2-158705 has disclosed the technique for acquiring pieces of subject distance information in a first distance-measuring mode in which the distances to points on the subjects are measured roughly, selecting the subject presenting the shortest distance, and measuring the distance only to the selected subject with high accuracy in a second distance-measuring mode, thereby improving the time-lag.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 63-131019 has disclosed a technique which is based on the assumption that, in active AF, the closest main subject is present at the place where the amount of light reflected from the projected light is the greatest and which omits distance-measuring calculations for parts where the amount of reflected light is small, thereby improving the time-lag.
Since all the conventional AF methods have used active AF, they have improved the time-lag remarkably. When they attempt to perform full-screen AF or the like, however, a set of floodlight elements and a set of light-receiving elements cannot avoid becoming very large, which is an obstacle to putting the device to practical use.
In contrast, passive AF would miniaturize the light-receiving elements much more than active AF, causing no obstacle to putting the device to practical use. Thus, passive AF is more suitable for wide-range multi-AF, such as full-screen AF, than active AF.
In this connection, Jpn. Pat. Appln. KOKAI Publication No. 62-103615 has disclosed the technique for performing rough correlation operation on distance-measuring areas, selecting one of the distance-measuring areas on the basis of the result of the operation, and then performing high-accuracy correlation operation on only the selected distance-measuring area, thereby improving the time lag by passive AF.
The rough correlation operation is performed by thinning out sensor data items, such as using every other sensor data item in calculations, but can never be omitted. Thus, although active AF has a higher efficiency of time-lag measures than passive AF, both types of AF produce the same effect.
A recently proposed solution to the problem of which of passive AF and active AF is more suitable for wide-range multi-AF, such as full-screen AF, is a distance-measuring method using hybrid AF. In hybrid AF, which is now in use, a steady-state light removing circuit for removing steady-state light is provided for each light-receiving element in a passive sensor. A passive operation is performed, when the steady-state light removing function is disabled, whereas an active operation is performed when the steady-state light removing function is enabled. Jpn. Pat. Appln. KOKAI Publication No. 10-336921 has disclosed such a steady-state light removing circuit. Products using hybrid AF are already available on the market.
To perform wide-range multi-AF, such as full-screen AF, time-lag measures are essential. For this reason, various devices have been thought out to avoid using high-speed, expensive CPUs and microcomputers at the sacrifice of cost. One of principal devices divides the process of measuring the distance in two:
the first half of the process for pre-distance measurement and the second half of the process for actual distance measurement.
The purpose of the pre-distance measurement is to measure the distance roughly in a short time and estimate the position of the main subject, whereas the purpose of the actual distance measurement is to limit time-consuming high-accuracy distance measurement to the necessary-minimum subjects on the basis of the result of the pre-distance measurement in the preceding process. Although the process of short-time pre-distance measurement increases, the time required to measure the distances to the subjects excluded is eliminated, which helps shorten the entire distance-measuring time.
More specifically, in one type of pre-distance measurement, light is projected onto subjects and the position of the main subject is estimated on the basis of the amount of reflected light.
In the estimating method, the subject with the largest amount of reflected light is generally judged to be the main subject present at the shortest distance.
Since estimates are accompanied by errors, it is common practice to prepare prospective main subjects in addition to the subject with the largest amount of reflected light.
If prospective main subjects were prepared on the very edge of the shooting screen in the estimating method, a problem would arise, judging from experience.
It is, therefore, necessary to take measures to deal with minor subjects on the periphery of the shooting screen in effecting wide-range multi-AF, such as full-screen AF, in the prior art.
The object of the present invention is to provide a high-accuracy distance-measuring system which estimates the position of the main subject correctly without being affected by minor subjects on the periphery of the shooting screen, operates at higher speed with less time-lag, assures a high reliability of the result of distance measurement, and suppresses a rise in manufacturing cost.
According to a first aspect of the present invention, there is provided a distance-measuring device comprising: at least a pair of integration-type light-receiving sensors for receiving the light from subjects via an optical system and producing a subject image signal; a light projecting section for projecting light onto the subjects; a steady-state light removing section for obtaining an image signal by removing the steady-state light component from the subject image signal the pair of integration-type light-receiving sensors produces, while the light projecting section is projecting light onto the subjects; a first distance-measuring section for measuring a subject distance on the basis of the image signal obtained by causing the steady-state light removing section to remove the steady-state light component from the subject image signal; a subject select section for causing the first distance-measuring section to make distance measurements for a specific time, performing operation on the resulting image signal using a specific correction function, and thereby selecting a subject whose distance is to be measured; and a second distance-measuring section for measuring the distance to the subject whose distance is to be measured which has been selected by the subject select section, on the basis of the subject image signal obtained by the pair of integration-type light-receiving sensors, while the light projecting section is not projecting light onto the subjects.
According to a second aspect of the present invention, there is provided a distance-measuring method comprising the steps of: causing at least a pair of integration-type light-receiving sensors to receive the light from subjects via an optical system and produce a subject image signal; causing a light projecting section to project light onto the subjects; causing a steady-state light removing section to obtain an image signal by removing the steady-state light component from the subject image signal the pair of integration-type light-receiving sensors produces, while the light projecting section is projecting light onto the subjects; causing a first distance-measuring section to measure a subject distance on the basis of the image signal obtained by causing the steady-state light removing section to remove the steady-state light component from the subject image signal; causing a subject select section to cause the first distance-measuring section to make distance measurements for a specific time, perform operation on the resulting image signal using a specific correction function, and thereby select a subject whose distance is to be measured; and causing a second distance-measuring section to measure the distance to the subject whose distance is to be measured which has been selected by the subject select section, on the basis of the subject image signal obtained by the pair of integration-type light-receiving sensors, while the light projecting section is not projecting light onto the subjects.
According to a third aspect of the present invention, there is provided a distance-measuring device comprising: at least a pair of integration-type light-receiving sensors for receiving the light from subjects via an optical system and producing a subject image signal; a light projecting section for projecting light onto the subjects; a steady-state light removing section for obtaining an image signal by removing the steady-state light component from the subject image signal the pair of integration-type light-receiving sensors produces, while the light projecting section is projecting light onto the subjects; a distance-measuring section for measuring a subject distance preliminarily on the basis of the image signal obtained by causing the steady-state light removing section to remove the steady-state light component from the subject image signal; and a subject select section for causing the distance-measuring section to make distance measurements for a specific time, performing operation on the resulting image signal using a specific correction function, and thereby selecting a subject whose distance is to be measured.
According to a fourth aspect of the present invention, there is provided a distance-measuring device comprising: a light projecting device for projecting light onto subjects; a light-receiving sensor which is composed of pixels and receives the light from the subjects; a select circuit for causing the light projecting device to operate and selecting at least one from the pixels according to the output from the light-receiving sensor; a corrective computing circuit for doing corrective calculations on the output value of the pixel selected by the select circuit using a specific correction coefficient; a decision circuit for determining a pixel corresponding to a main subject according to the output value obtained through corrective calculations by the corrective computing circuit; and a subject distance information circuit for calculating subject distance information using the output value of the pixel determined by the decision circuit.
It is desirable that the specific correction function should be a function which uses the position of the subject image signal, the focal length of the optical system, and the shooting screen aspect ratio in the optical system as inputs.
Furthermore, it is desirable that the specific correction function should be a function which outputs a coefficient corresponding to the probability of presence of the subject.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.